Lightweight transmission line packages can be installed
in a duct by a procedure known as `fibre blowing`. The technique involves blowing compressed gas along the duct into which the transmission line is to be installed and feeding the transmission line package into the duct at the same time so that the package is urged along the duct by viscous drag of the gas flow. The technique is of particular importance to optical fibre transmission lines which can be damaged by the tension that is produced in pulled installation techniques. In new installations ducts for transmission lines are usually underground or incorporated into the structure of a building and it is difficult and/or costly to install new ducting at a subsequent date to cope with increased demand for lines or for replacement lines. Therefore when ducts are first installed additional ducts for future use will be laid, and it may be several years after the duct installation that fibre packages are actually blown through some of these ducts. A degree of over capacity may also mean that there is a choice of which duct to use. A problem that arises with this system is that, during the time between duct laying and installation of fibre, damage may occur to one or more of the ducts which renders them temporarily or permanently unsuitable for use. For example, a complete blockage caused by the duct being crushed would make it impossible for the fibre to be installed, a partial blockage may make installation very difficult and puncture may cause such a loss of the blowing gas that installation is slow or impossible and also exposes any installed fibre to potential damage from water flooding into the duct. Thus it is desirable to be able to ascertain the state of a duct prior to installation, and in the event of a fault to know its type and location. There are also occasions when the continuity or state of a duct containing an installed transmission line may require monitoring.